Sysreqs: do not call pre_install and post_install if not packages to install?

[gaborcsardi]

Although there is at least one rule that'll not work then, for chrome, because the pre-install does all the work.

OTOH if we don't call them, then pak will work on RHEL containers that are not registered, as long as the sysreqs are already installed or if they are in the ubi or epel repos.